summary = "A bench power supply is one of the essential tools of any electronics hobbyist.Although you can buy a such a unit for less that € 50, it's way more fun to build one yourself."
Let's walk through these quickly. All `COM` pins are common ground and can be used
with any supply voltage.
* Pins 1,2, 12 and 13 provide 3.3V
* Pins 4, 6, 21, 22 and 23 provide 5V
* Pins 10 and 11 provide 12V
* Pin 14 provides -12V
* Pin 9 provides 5VSB, always available when the supply is connected to mains
* Pin 16 is the power switch, connect it to GND to turn the supply on.
* Pin 8 (PWR_ON) supplies 5V when the power supply is on _and_ providing stable voltages.
* Pin 20 is marked as _not connected_. In the past the ATX spec placed an optional -5V here, but has since been removed all together. Don't rely on -5V to be available.
The most important thing to note here is the `PWR_ON` pin. It supplies 5V when
the supply is providing a stable output. `PWR_ON` does not come on instantly, as
it takes some time for the ATX supply to stabalize. Although this process is quite
fast, there is a noticable delay of about half a second between switching the supply
on and the `PWR_ON` going high.
## Schematic
All the hard work of converting mains 230VAC to a more suitable 3.3/5/±12V is done by
the ATX supply. The only custom things left to do are:
* Accept a 20 or 24 pin ATX connector
* Provide a connection for the power-on and standy-by LEDs
* Provide a connection for a power switch
* Provide connections for 3.3/5/±12V outputs and GND
* Limit current to 2.5A for each output voltage
The schematic for this is rather straight forward:
![Schematic](/img/voltmeister-100-revc.png "VoltMeister 100 Rev C Schematic")
The power LED is directly hooked up to pin 8, `PWR_ON` with a series resistor to limit
the current through the LED and make it not annoyingly bright. The same goes for the stand-by LED.
The power switch is directly connected to pin 16 and GND, pulling pin 16 low when switched on.
The four output voltages, together with GND, are routed to output pins so they're easy to hook
up to the actual banana sockets in the case.
Each supply voltage is fitted with a 2.5A resettable fuse (or PTC). Anything up to 2.5A is fine,
above that the fuse will start to act as a circuit breaker. When the faulty situation is resolved,
the PTCs will reset and you use the supply again.
_Note: you may notice there is a 2.5 amp PTC on the -12V output. That's weird because the
power supply limits this voltage to 0.3amps anyway. Keep in mind that other power supplies
may be rated for higher current on the -12V rail and will need this PTC to keep me safe._
## PCB
Because the ATX connector (MOLEX 39-28-8240) uses a 4.2mm pitch spacing it does not fit
on a protoboard. So, let's design a PCB! Here's my the C revision of my PCB design:
![VoltMeister 100 PCB](/img/voltmeister-revc-pcb.png "VoltMeister 100 Rev C PCB")
I've created wide traces for the supply voltages, as up to 5A needs to be able to
flow through them. (5A is the trip-current of the PTCs). Each voltage, together with
GND is exposed through screw terminals. There are pinheads for the LEDs and the switch.
I've opted for SMD parts because I wanted like to give SMD soldering a try.
## Lesson learned
* ATX power supplies are easy to work with, but also quite powerful. Be very careful if you open one up.
* I should mark the +/- for the LED and terminals more clearly
* Consider the connection points on the PCB, it turned out that it would have been easier if the LED/Button connections were on the same side as the voltage output terminals.